Type relationship as list

[tiangolo]

✨ Type relationship as list.

Currently, the plugin types relationships with uselist as Iterable[X], but these relationships actually expose a list with list semantics, including .append(), assignments, etc.

This PR updates the plugin to type those relationships with List[X] instead of Iterable[X].

Ref: https://docs.sqlalchemy.org/en/13/orm/relationship_api.html#sqlalchemy.orm.relationship.params.uselist

Ref of usage with assignment: https://docs.sqlalchemy.org/en/13/orm/tutorial.html#working-with-related-objects
Ref of usage with append: https://docs.sqlalchemy.org/en/13/orm/tutorial.html#building-a-many-to-many-relationship

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Note: I'm by no means expert in mypy internals and its plugins, I'm not 100% sure the usage of ctx.api.named_generic_type('builtins.list', [new_arg]) is correct here, but I see that seems to be how it's used by mypy: https://github.com/python/mypy/blob/master/mypy/plugins/ctypes.py#L158

[lukaszdudek-codibly]

Fixes #152

[lukaszdudek-codibly]

Also, this PR doesn't take into consideration #107 (not that it has to).
IMHO List should be the default type for relationship fields due to argumentation provided above by @tiangolo . Then, support for other values of collection_class can be added.

[lukaszdudek-codibly]

I've just found #82, which this PR extends by adding tests.
I guess that the topic is a pain for many people 😄

[shawnwall]

yes, this topic is a pain. is anything else needed before this can be merged?

thanks @tiangolo for raising/fixing.

[chardigio]

Thanks for this! Just figured I'd bump @shawnwall 's comment, is anything else needed before this can be merged?

[ilevkivskyi]

OK, I will merge this now. We can revisit then if this will cause issues with internal repos.

[tiangolo]

Awesome! Thanks @ilevkivskyi ! 🚀